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Eastern Section Meeting

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The Precambrian basement of Eastern Ontario: Insights from a new aeromagnetic survey

Abstract

The Precambrian basement beneath the Paleozoic of eastern Ontario is poorly known due to low resolution aeromagnetic coverage (800 m line spacing) and limited drilling. To improve our knowledge, in late 2013, 34,724 line km of aeromagnetic data were collected at 400 m line spacing over a 12,515 km2 area of eastern Ontario east of longitude 76.51W to the Quebec border. These data were recently released as Ontario Geological Survey Geophysical Dataset 1075. This presentation summarizes both the results of the survey and the preliminary interpretation of the underlying basement geology, based on tracing known surface geology beneath the Paleozoic, which is assumed to be magnetically transparent. Eastern Ontario is underlain by Mesoproterozoic metasedimentary and metaplutonic rocks of the Grenville Province: the granulite facies Frontenac-Adirondack Belt (FAB) to the southeast and the lower amphibolite Sharbot Lake domain to the northwest, separated by the ca. 1160 Ma Maberly shear zone (MSZ). A 30 km wide zone of linear magnetic trends on the Frontenac side of the MSZ is interpreted as a high-strain zone parallel the boundary. The magnetic grain of FAB is characterized by magnetic lows (marbles, quartzites, some gneisses) and ovoid magnetic highs (ca. 1160 & 1080 Ma plutons), which can be reliably traced beneath thin (<400 m) Paleozoic cover as far east as the east-trending Gloucester fault, across which thickness of the Paleozoic increases abruptly (>800 m), resulting in a considerably subdued magnetic grain. East of the fault, interpretation of the basement geology is less reliable, although a possible Monteregian (Mesozoic) intrusion and a basement horst occur near the Quebec border north of Cornwall. Several large dikes, possibly related to the Grenville or Rideau dike swarms, are present beneath the Paleozoic north of Gananoque. Surprisingly, few faults and lineaments identified in the basement propagate into the overlying Paleozoic strata. Furthermore, many mapped faults in the Paleozoic are not obviously present in the basement magnetic pattern; for example west to northwest faults near Ottawa strike at a high-angle to ductile shear zones in the basement. In contrast, northeast-trending faults along the St. Lawrence River are subparallel to basement ductile shears zones, which may have been reactivated. The new survey should prove invaluable in exploration for both groundwater and other subsurface resources in eastern Ontario.